1. Field of the Invention
The present invention relates to a dynamic pressure bearing device in which a dynamic pressure is generated by a lubricating fluid to support a shaft member or a bearing member one of which is relatively rotated.
2. Description of the Related Art
In recent years, various types of dynamic pressure bearing devices have been proposed or a device rotating various types of rotary bodies such as a polygon mirror, a magnetic disk and an optical disk. In such a dynamic pressure bearing device, the outer peripheral face of a shaft member and the inner peripheral face of a bearing member face each other via a minute gap. One of the faces is armed with a dynamic pressure generating part. A lubricating fluid such as air or oil in the minute gap is pressurized by a pumping operation at the dynamic pressure generating part during rotation. Either the shaft member or the bearing member is relatively rotatably supported by the dynamic pressure of the lubricating fluid.
Some dynamic pressure bearing devices are provided with grooves in a herringbone or spiral shape as a means for generating dynamic pressure. On the other hand, especially for journal bearing devices, step-formed and dynamic pressure bearing devices without using herringbone or spiral dynamic pressure generating grooves have been proposed.
FIG. 26(A) is a transverse cross-sectional view of a conventional taper-formed dynamic pressure bearing device, and FIG. 26(B) is an explanatory development view of a dynamic pressure generation part which is formed in the dynamic pressure bearing device shown in FIG. 26(A). In the dynamic pressure bearing device 30xe2x80x2 shown in the figure, five dynamic pressure generating parts 31xe2x80x2 are formed along a circumferential direction on the inner peripheral face of a bearing sleeve 15xe2x80x2 (bearing member) which encloses a rotation shaft 21 (shaft member) via a minute gap 32xe2x80x2. Each of the dynamic pressure generating parts 31xe2x80x2 is composed of a protruded part 37xe2x80x2 which is formed so as to make the radial dimension of the minute gap 32xe2x80x2 the narrowest, a recessed isolation groove 38 which is formed of a depth of 23 xcexcm over about 7.5xc2x0 of the circular angle, and a tapered part 36xe2x80x2 which makes the radial dimension of the minute gap 32xe2x80x2 continuously vary about 6 xcexcm between the recessed isolation groove 38 and the protruded part 37xe2x80x2. When the rotation shaft 21 is rotated in the direction of arrow xe2x80x9crxe2x80x9d, the lubricating fluid such as air or oil is pressurized to generate a desired dynamic pressure in the minute gap 32xe2x80x2 formed between the rotation shaft 21 and the bearing sleeve 15xe2x80x2.
However, the dynamic pressure bearing device 30xe2x80x2 provided with the recessed isolation grooves 38 for canceling negative pressure has a low rigidity, which causes the following problems. First, the deflection of the rotation shaft 21 is liable to occur when disturbances are applied to the shaft 21 at a low speed rotation. In addition, since the rotation speed required to float by the dynamic pressure is high, a metal-to-metal contact occurs between the rotation shaft 21 and the bearing sleeve 15xe2x80x2 at a low speed rotation of about 5000 rpm, which may cause abrasion. Accordingly, the long life of the dynamic pressure bearing device 30xe2x80x2 cannot be attained.
In order to solve such problems, it may be inconceivable to make the gap between the rotation shaft 21 and the bearing sleeve 15xe2x80x2 narrower by strictly controlling the dimensional tolerances of the rotation shaft 21 and the bearing sleeve 15xe2x80x2. However, such a countermeasure is unfavorable because the components"" cost and assembling cost are increased. It is also conceivable to make the diameter of the rotation shaft 21 larger to increase its peripheral velocity, but it is also unfavorable because such a countermeasure causes an increase in cost. It is also conceivable that a high abrasion resistance material is used for the rotation shaft 21 and the bearing sleeve 15xe2x80x2, but it also causes an increase in the cost of the components.
Further, when the recessed isolation grooves 38 are provided, the contraction percentage of the lubricating fluid in the minute gap 32xe2x80x2 becomes so large that the lubricating fluid is unable to get into the small gap 32xe2x80x2 and may leak out in the axial direction.
The recessed isolation grooves 38 are preferable to be formed as deep and narrow as possible to obtain satisfactory dynamic pressure characteristics and normally formed with a depth of 20 xcexcm or more. Accordingly, the cutting work is performed to form deep and narrow grooves in the manufacturing process of the dynamic pressure bearing device 30xe2x80x2. The reason is that it is difficult to form deep and narrow recessed isolation grooves 38 by another method. Accordingly, the conventional dynamic pressure bearing device 30xe2x80x2 requires an additional production process different from the normal manufacturing process. Workability of cutting the work is not satisfactory for a dynamic pressure bearing device. This means the cutting work is not suitable to make a deep and narrow recessed isolation grooves for a dynamic pressure bearing device, although only a simple deep and narrow recessed isolation groove can be formed by cutting work. Therefore, much caution and slow handling are required to form a satisfactory isolation groove for a dynamic pressure bearing device, which resulting in reduced productivity due to low efficiency.
In view of the problems described above, it is an advantage of the present invention to provide a dynamic pressure bearing device capable of generating satisfactory dynamic pressure even if the recessed isolation grooves are not formed.
In order to achieve the above advantage, according to the present invention, there is provided a dynamic pressure bearing device including a plurality of dynamic pressure generation parts on a peripheral face either of a shaft member or a bearing member. The dynamic pressure generation parts are respectively formed in a circumferential direction at equal angular intervals and extended in an axial direction so as to form a protruded part which makes the dimension of a minute gap the smallest and a recessed part which makes the dimension of the minute gap the largest. The dynamic pressure bearing device also includes a perfect circle part formed on the peripheral face at least on a shaft end side at a region which is adjacent to the dynamic pressure generation part where the protruded part and the recessed part are formed. The dimension of the minute gap in the perfect circle part is set to be the same as that in the protruded part.
The dynamic pressure bearing device according to the present invention is provided with the protruded part and the recessed part, but is not provided with a conventional recessed separation groove. Accordingly, the shaft member or the bearing member produced by, for example, a sintered mold body does not require the recessed separation groove by means of cutting work and therefore productivity of the dynamic pressure bearing device can be improved. In addition, since the recessed separation groove is not formed, the leakage of the lubricating fluid in the axial direction can be restrained.
In accordance with an embodiment of the present invention, the region which is adjacent to the dynamic pressure generation parts on the shaft end side is formed in the perfect circle part. Therefore, the rigidity in the center direction can be made larger and the deflection is hard to occur even when disturbances are applied at a low speed rotation. In addition, since the speed of rotation by which floating begins due to the dynamic pressure can be made lower, a metal-to-metal contact is hard to occur between the shaft member and the bearing member even at a low speed rotation, and hence the life of the dynamic pressure bearing device can be made longer. Therefore, the dimensional tolerance of the shaft member and the bearing member does not need to be strict so as to improve its rigidity, and the diameter of the shaft member is not required to be made larger to increase its peripheral velocity Also, it is not necessary to use an expensive material of high abrasion resistance for the shaft member and the bearing member, and therefore the dynamic pressure bearing device with satisfactory characteristics can be provided without increasing cost.
In this configuration, the perfect circle part is preferably formed on the output end side to which a rotational load is coupled. In the shaft member or the bearing member, its center of gravity is shifted on the side to which the rotational load is coupled and thus the deflection is easy to generate. On the contrary, when the perfect circle part is formed on the side to which the load is coupled, the perfect circle part is effective to prevent the deflection.
Preferably, the ratio of the axial length of the perfect circle part and that of the part where the shaft member and the bearing member oppose to each other is preferably set in the range of 0.03 to 0.3. When the ratio is less than 0.03, the effect due to the perfect circle part is made smaller and, on the other hand, when the ratio is more than 0.3, the region of the dynamic pressure generation part is decreased and thus the rigidity is lowered.
Preferably, the edge portion of the recessed part which is located on the shaft end side, to which a rotational load is fitted, is formed oblique so that the corner part to which the lubricating fluid flow in when the shaft member is relatively rotated with the bearing member is formed as an acute angle.
In accordance with another present invention, there is provided a dynamic pressure bearing device including a plurality of dynamic pressure generation parts on a peripheral face either of a shaft member or a bearing member. The dynamic pressure generation parts are respectively formed in a circumferential direction at equal angular intervals and extended in an axial direction so as to form a protruded part which makes the dimension of a minute gap the smallest and a recessed part which makes the dimension of the minute gap the largest. Further, the edge portion of the recessed part is formed oblique so that the corner part to which the lubricating fluid flows in when the shaft member is relatively rotated with the bearing member is formed as an acute angle.
In this configuration, the edge portion on the shaft end side of the recessed part is formed oblique as the corner portion where the lubricating fluid flows into the recessed part is formed so as to be an acute angle. Therefore, when the shaft member is relatively rotated with the bearing member, the lubricating fluid is sent to the center side in the axial direction by the wall face of the edge portion of the recessed part which is formed oblique. Accordingly, the lubricating fluid is pressurized on the center side in the axial direction and thus a large dynamic pressure is generated. The rigidity can be increased and the deflection is hard to occur even when disturbances are applied at a low speed rotation. In addition, since the speed of rotation by which floating begins due to the dynamic pressure can be made lower, a metal-to-metal contact is hard to occur between the shaft member and the bearing member even at a low speed rotation, and hence the life of the dynamic pressure bearing device can be made longer. Therefore, the dimensional tolerance of the shaft member and the bearing member does not need to be strict to improve its rigidity, And the diameter of the shaft member is not required to be made larger to increase its peripheral velocity. Also, the dynamic pressure bearing device with satisfactory characteristics can be provided without increasing cost.
When a step-formed dynamic pressure bearing device is constituted, the protruded part and the recessed part are preferably formed in a step shape where the protruded part and the recessed part are adjacent to each other in the circumferential direction. By this constitution, the shaft member or the bearing member provided with the dynamic pressure generation parts can be produced at a low cost and thus cost reduction of the dynamic pressure bearing device can be obtained. That is because the shaft member or the bearing member provided with the dynamic pressure generation parts can be produced from the sintered compact which is formed from powder including metal by using a die and the die capable of being divided vertically can be utilized to perform the molding. In addition, step-shaped recesses and projections can be also formed on the surface of a die for molding by etching.
Preferably, the dynamic pressure generation parts are formed at three to five regions along the circumferential direction. Also, the ratio of the circular angle of the region where the protruded part is formed with respect to the circular angle of the region where the dynamic pressure generation part is formed is preferably set between 0.2 and 0.5. According to these constitutions, since the rigidity in the center direction can be made larger, deflection is hard to occur even when disturbances are applied at a low speed rotation.
Preferably, the shaft member or the bearing member provided with the dynamic pressure generation parts is made of the sintered compact which is formed from powder including metal. By this constitution, the shaft member or the bearing member can be produced at a low cost and thus cost reduction of the dynamic pressure bearing device can be attained.
According to one embodiment of the present invention, the lubricating fluid is air.